Braced Excavation - Pressure Diagrams

[RFreund]

See attached Sketch.

My questions are as follows:

[ol 1]
[li]Are the diagrams correct? Even for long term conditions?[/li]
[li]If you have a anchor top and bottom vs only at the top of a retaining wall say steel or concrete, the pressure distribution changes correct?[/li]
[li]If it does change, is this because of the 'rigidity' of the anchor vs the embedded toe which may deflect or move some (presumably more than the anchor)? Therefore there is some sort of soil 'bridging' or stiffness attracts force concept?[/li]
[li]The 'sand' pressure diagrams would also apply to granular materials, correct? i.e. any "phi" soil.[/li]
[li]Are there anchors which would be considered flexible, where you would consider more of a triangular distribution? i.e. duckbill anchor, helical, deadman etc.[/li]
[/ol]

EIT

http://www.HowToEngineer.com

 

[PEinc]

The answers you are looking for are too much to type in this forum. If you wish, call me to discuss this. I can talk much faster than I can type.

http://www.PeirceEngineering.com

 

[RFreund]

PEinc - Thanks, you and FixedEarth are always willing to help, I appreciate that.

EIT

http://www.HowToEngineer.com

 

[FixedEarth]

RF;

1-The older texts give an option of triangular earth pressure distribution for cantilever walls and one level anchor walls. However, in current practice, for one or multilevel, we use a variation of trapezoidal earth pressure distribution. The triangular earth pressure is applicable only to cantilever walls.
2-The trapezoidal earth pressuredistribution is a function of wall height, soil friction angle, cohesion and unit weight. The location of anchor elevation and the number of anchors only influence the induced shear, moment and deflections in the wall but not the design earth pressure.
3-N/A.
4-Yes it is applicable to all granular soils.
5-Some anchors are active like prestressed tieback anchors and some are passive like soil nail anchors. For anchor design, you need to go beyond the active failure plane, have enough length to meet slope stability rotational f.s. and have enough capacity to exceed the design load.

On your lower diagram, the maximum moment is usually at 3 to 5 ft below the dredge line. So it is better to have one continuous pile or embed the burried portion inside a drilled pier - like soldier beams. You don't want to have a pin connection or helical pile at the location of highest bending moment.

For more information, refer to Blum's method covered in A. R. Jumikis's Foundation Engineering book-2nd edition. Blum has two methods for cantilever walls and one method for anchored walls. Between those three method you can obtain your earth pressures for sheetpiles, soldier beams, secant pile wall, tangent pile wall, etc.

 

[PEinc]

If the retained soils are soft or if the single tier brace is very high on the wall, I would use a triangular pressure distribution.

http://www.PeirceEngineering.com

 

[RFreund]

FE - Thanks for the reply.

On your lower diagram, the maximum moment is usually at 3 to 5 ft below the dredge line. So it is better to have one continuous pile or embed the burried portion inside a drilled pier - like soldier beams. You don't want to have a pin connection or helical pile at the location of highest bending moment.

Makes sense.
Just an FYI (and without getting too specific with this as I'm trying to make sure I understand the concept first) - In the my lower diagram I am trying to get away with less embedment of the actual wall and thus was thinking of using a vertical helical pier with a lateral anchor

PEInc - I still plan to call at some point but I want to make sure I have done my due diligence here and I need some time...

EIT

http://www.HowToEngineer.com

 

[mudman54]

don't forget to calculate the settlement behind the wall !